In hair dyeing process, an oxidative hair dye composed of a first part containing an alkali agent, a precursor and a coupler, and a second part containing hydrogen peroxide, has been generally used.
Preferably, such a hair dye composition has superior dyeing properties, in order to dye the hair in a shorter time and dye the hair more vividly. As a convenient method for enhancing the hair dyeing properties, for example, a method of increasing the concentration of incorporated dye is used. However, if the dye concentration is merely increased, the tinge of color obtained is altered, further an adverse effect may be brought to the stability of formulation. Thus, there have been limitations on the increase in the concentration of dye. Furthermore, a method of increasing the amount of incorporation of the alkali agent or the oxidizing agent and enhancing bleaching properties, to thereby make the change of the dyed-hair color more conspicuous, is also used in combination therewith. However, there have also been limitations in increasing the amount of incorporation of the alkali agent or oxidizing agent because there are problems of, for example, increase of hair damages or the unnatural finish with reduced moist feeling or depth.
In order to solve such problems, methods of using various novel precursors or couplers, or methods of using novel direct dyes have been proposed. However, when these novel dyes are used, enormous efforts are required in adjusting the tinge of color, and therefore, there is a demand for technologies of enhancing dyeing properties using only the conventional dyes which have been widely used, without using novel dyes.
Examples of such technologies that have been proposed include a technology of enhancing dyeing properties by using a hair dye composition containing various cationized protein hydrolysates (Patent Document 1); a technology of enhancing the dyeing properties and bleaching properties at the same time, and thus obtaining higher hair dyeing effects, by using a hair dye composition containing a protein hydrolysate or a derivative thereof, and monoisopropanolamine (Patent Document 2); and the like. In these technologies, use of the hydrolysates of various animal proteins or plant proteins is suggested, however, the composition of the protein hydrolysate varies with the protein source, and moreover, the compositions of different production lots are not always constant. For that reason, the effects described above, provided by the technologies mentioned above, are easy to vary, and are not sufficiently satisfactory.
Patent Document 1: JP-A-H01-275519
Patent Document 2: JP-A-2000-336020